Tag Archives: crystallography

Left: Van Gogh painting “Wheat Stack under a Cloudy Sky” (Kröller-Müller Museum, Netherlands). The paint sample area is indicated by a white circle. Upper right: Detail of the painting in the sample area, lower right: Detail of the paint sample (picture: Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)

Why Van Gogh’s paintings are fading to white

Belgian scientists have revealed a refined explanation for the chemical process that’s currently degrading Vincent van Gogh’s famous paintings, which are losing their bright red. Like other old paintings, van Gogh’s works are losing their saturated hue because of the interaction between red led and light. Using sophisticated  X-ray crystallographic methods, the researchers identified a key carbon mineral called plumbonacrite in one of his paintings, which explains the process even better.

Left: Van Gogh painting “Wheat Stack under a Cloudy Sky” (Kröller-Müller Museum, Netherlands). The paint sample area is indicated by a white circle. Upper right: Detail of the painting in the sample area, lower right: Detail of the paint sample (picture: Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)

Left: Van Gogh painting “Wheat Stack under a Cloudy Sky” (Kröller-Müller Museum, Netherlands). The paint sample area is indicated by a white circle. Upper right: Detail of the painting in the sample area, lower right: Detail of the paint sample (picture: Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)

All paints are made up of three key parts: the vehicle (usually water), the pigment (the stuff that gives matter color – usually mined from the earth in the form of clay or mineral or even plants, but also synthetic form), and a binder (otherwise you’d just have colored water – typically chalk is used). Red lead (minium, or lead (II,IV) oxide) is a lead oxide whose composition is Pb3O4 and whose color varies over time. It’s been a favorite pigment for thousands of years. In fact, it can still be found in the old cave paintings some 40,000 years old. Of course, it degrades over time darkening as the red lead pigment is converted to plattnerite (beta-lead dioxide) or galena (lead sulfide). At other times, the color will lighten or bleach due to the conversion of red lead to lead sulfate or lead carbonate.

A team led by Koen Janssens at the University of Antwerp investigated what makes van Gogh’s paintings turn white by taking a microscopic sample from “Wheat Stack under a Cloudy Sky”, one of his famous work, and subjecting it to crystallographic analysis. X-ray powder diffraction mapping and tomography techniques were employed to determine the spatial distribution of the various crystalline compounds found throughout the sample. They eventually found an unexpected compound, the very rare lead carbonate mineral called plumbonacrite (3 PbCO3·Pb(OH)2·PbO).

“This is the first time that this substance has been found in a painting from before the mid twentieth century,” reports Frederik Vanmeert, first author of the paper. “Our discovery sheds new light on the bleaching process of red lead.”

Considering this latest finding, the Belgian researchers proposed a chemical reaction pathway of the red lead under the influence of light and CO2, which ultimately altered the pigment and caused a color change in the painting. As light hits the paint (red lead and other pigments), the incoming energy causes electrons to move from the valance band to the conducting band in red lead, which is a semi-conductor. This reduces the red lead to PbO, which reacts with other products formed by the reaction of CO2 from the air with the degrading binding medium. Ultimately, this forms plumbonacrite  as an intermediate that is converted to hydrocerussite and then to cerussite (lead carbonate). All these products are white, hence the lower saturation. The findings were reported in  Angewandte Chemie.

Art aficionados shouldn’t fret too hard, though. Van Gogh’s paintings are still marvelous, despite more than a hundred years since the Dutch painter made his first stroke on the canvas. Museums give great care and employ special conservation methods to keep the old masters’ work bright and vibrant for hundreds of years to come.

DNA pattern

Liquid DNA crystals imaged in stunning timelapse

DNA pattern

DNA is widely recognized by its double helix, but if you look at the molecule through a microscope you might be disappointed. That’s because the double helix is an atomic model, and you’d need a really powerful microscope to see the helix. On a grander scale, DNA can take some interesting shapes. Take for instance these images of liquid DNA crystallization taken by artist and biochemist Linden Gledhill, which he then stitched together to form a time-lapse video. Imaging DNA is no easy task, and through this mesmerizing display Gledhill hopes to inspire other scientists and laymen as well to delve deeper into DNA and its significance.

The work was commissioned by the MSSNG project, a program launched by the advocacy organization Autism Speaks. The aim is to sequence the DNA of 10,000 families affected by autism and then publish all the data freely in a open-source fashion.

DNA crystalization

To capture the stunning displays of DNA crystallization, Gledhill used a professional lab microscope that magnifies up to 1,000 times. But what’s the deal with all those colours? The trick lies in using polarized light. The light thus gets twisted by the DNA crystals, causing an interference in the spectrum.

“It’s actually very cool because few people have really seen images like these before our research groups,” explains Gledhill. “When people see them they ask me, ‘What is that?’ They have no idea and are quite surprised it’s DNA.”

As water evaporates from the edges of DNA samples placed between two glass slides, the structure gradually crystallizes. The dark areas are where there is liquid and no structure but as the molecules become better aligned we see these vivid colors. The whole process was captured in the video you can watch below.

The X-ray snapshots in the figure show the atomic arrangement of the molecule being brominated before, during, and after the reaction. Photo: Fujita et al/JACS

X-rays image atoms during chemical reactions for the first time

Since its advent some 100 years ago, crystallography has become one of the most important processes in chemical research and development. It involves bombarding a material with X-rays to produce a diffraction pattern as they reflect off the sample. The pattern can be used then to directly determine the atomic structure of the crystal. Using this technique, the structure of DNA was first obserbed, along with that of diamond, table salt, penicillin, numerous proteins, and entire viruses.

Crystallography works for only still structures, yet if Makoto Fujita at the University of Tokyo is correct, then a refined process can be used to image atomic arrangements as chemical reactions happen in real time. This means nothing short of crystallography 2.0 – similar to the technological jump from still photography to motion picture video recording.

Fujita and colleagues studied how a catalyst – a molecule that accelerates a chemical reaction without actually reacting with the elements involved in it – called palladium worked its magic in a reaction where it accelerates the attachment of a bromine atom to a larger molecule. This reaction was carried out in a solution, however modern crystallography can not provide snapshots of atomic structures of molecules moving in a solution. The researchers thus had to employ a trick.

The X-ray snapshots in the figure show the atomic arrangement of the molecule being brominated before, during, and after the reaction. Photo: Fujita et al/JACS

The X-ray snapshots in the figure show the atomic arrangement of the molecule being brominated before, during, and after the reaction. Photo: Fujita et al/JACS

The scientists trapped the catalyst and reacting molecules in a cage, before taking X-ray snapshots during the reaction. This proved to be key for their experiments since it made the molecules still for enough time to allow X-ray imaging capture. This helped Fujita and colleagues better explain and determine how the palladium catalyst played its part in the said reaction. Most importantly, however, the experiment demonstrates a new way to use crystallography to image the structure of changing compounds.

Findings appeared in the Journal of American Chemical Society.